This study tested the hypothesis that CRHRs (
corticotropin-releasing hormone receptors) in the nucleus of the solitary tract (NTS) contribute to the
hypertension induced by intermittent
hypoxia (IH) exposure in rats. Initial studies using in situ hybridization revealed low
mRNA level of CRHR1 (CRH type 1 receptor) but high
mRNA level of CRHR2 (CRH type 2 receptor) in the NTS.
Calcium imaging studies on NTS slice preparations using Fura-2-acetoxymethyl
ester demonstrated that CRH induced a transient increase of intracellular
calcium level. The CRH-induced
calcium response was reproduced in the presence of TTX (
tetrodotoxin) but was abolished by depletion of extracellular
calcium or by the
L-type calcium channel blocker
Nifedipine. The CRH-induced
calcium influx was attenuated by the CRHR2 antagonist
K41498 but not by the CRHR1 antagonist NBI-35 965.
Calcium influx can be induced by the CRHR2 agonist
Urocortin II but not by the CRHR1 agonist Stressin 1. IH exposure did not affect CRHR1
mRNA level but significantly decreased CRHR2
mRNA level and the CRH-induced
calcium influx in the NTS. Further in vivo studies showed that intra-fourth ventricle infusion of
K41498 did not affect the basal blood pressure but significantly attenuated the IH-induced
hypertension; intra-fourth ventricle infusion of
Urocortin II significantly increased basal blood pressure and exacerbated the IH-induced
hypertension. Collectively, these results suggest that CRHR2 in the NTS contributes to the IH-induced
hypertension; downregulation of CRHR2 and CRHR2-mediated
calcium influx in the NTS may serve as an adaptive response to protect against the IH-induced
hypertension.